1. Field of the Invention
The present invention relates to an apparatus for driving a control rod for adjusting power from a reactor, preferably adapted for a boiling water reactor.
2. Description of the Related Art
In general, the basic operation for controlling power from a nuclear reactor is an adjustment of a reactivity thereof. By adequately controlling the quantity of the reactivity, the reactor plant can totally be controlled. In many reactors, control of the reactivity is performed by inserting or withdrawing a control rod, in which a neutron absorber is enclosed, to and from a reactor core.
In a boiling water reactor (BWR), four fuel assemblies are disposed around a cross-shaped control rod to form one unit, and a plurality of the thus-arranged units are disposed so as to constitute the reactor core. The reactivity of the BWR is controlled by withdrawing or inserting the control rod from or into the reactor core. The control rod is inserted/withdrawn by a control rod driving apparatus connected to the control rod.
FIG. 10 is a view which illustrates an example of the structure of a conventional control-rod drive apparatus. The control rod driving apparatus 1 is inserted into a housing 3 which is formed integrally with a reactor pressure vessel 2 by welding. The control rod driving apparatus 1 has an electric motor 4 at the lower end thereof. A ball spindle 5, the rotations of which are controlled by the electric motor 4, is supported by a bearing 6 and a roller 7 through a rotational shaft 21. A ball nut 9, the rotations of which are inhibited by a groove, not shown, vertically formed in the inner surface of a guide tube 8, is attached to the ball spindle 5 by means of a thread. A connection pipe 10 having the lower end supported by the ball nut 9 establishes the connection with a control rod 11 to be inserted/withdrawn from a reactor core, not shown, of the reactor. The connection pipe 10 has a structure that its rotation is inhibited similarly to the ball nut 9. Furthermore, a lower guide roller 12 is disposed adjacent to the lower portion of the connection pipe 10 so that movement of the connection pipe 10 in the circumferential direction is restricted and its axial directional movement is smoothened. The lower end of the guide tube 8 is placed on a cylinder member 51 connected to the housing 3 through a spool piece 20, the guide tube 8 having the top end to which a damper 13 is attached. The damper 13 is supported by an upper guide 15 through a disc spring 14 so as to be capable of moving upwards a small distance. The upper guide 15 is attached to the reactor pressure vessel 2 through a damper sleeve 16. In addition, outward leakage of reactor water is prevented by a shaft sealing packing 17 disposed between the ball spindle 5 and a cylinder member 51 so that a water-tight seal is realized.
In the thus-constituted control rod driving apparatus 1, when the ball spindle 5 is rotated due to rotations of the electric motor 4, the ball nut 9 allowed to engage with the ball spindle 5 is permitted to be moved only in the axial direction. Therefore, the connection pipe 10 mounted on the ball nut 9 follows the movement of the ball nut 9, causing the control rod 11 connected to the connection pipe 10 to be moved vertically.
If the control rod 11 is rapidly inserted (hereinafter called "scram") during an emergency for the reactor, water accumulated in an accumulator, not shown, is passed through a scram-water injection pipe 18 so as to be introduced into the guide tube 8. As a result, the connection pipe 10 is rapidly pushed in the upper direction so that the scram is performed.
Thus, the control rod driving apparatus 1 is, as described above, driven by the electric motor operated in a usual operation and by the hydraulic pressure used in the case where the scram is performed.
The positions of the connection pipe 10 and the control rod 11 are maintained by the maintaining torque of the electric motor 4 and the friction of the shaft sealing packing 17. In the case where the scram-water injection pipe 18 is broken, for example, it may be considered that hydraulic pressure for pushing the connection pipe 10 in the downward direction acts. Accordingly, an electro-magnetic brake 19 is attached below the electric motor 4.
Since the body of the control rod driving apparatus has no sliding and contact elements such as the piston seal, it has been considered that it has no element which must be periodically changed in a period of forty years which is the life of the reactor. That is, a substantially maintenance-free state has been achieved for the body of the control rod driving apparatus.
Since the shaft sealing packing 17 is gradually degraded due to sliding and high temperature environment in the shaft sealing portion, periodical change is required. The shaft sealing packing 17 is periodically changed in a procedure such that initially the electric motor 4 is removed, and the spool piece 20 accommodating the shaft sealing portion is removed. Then, the spool piece 20 is decomposed, and the shaft sealing packing 17 is changed. Assembly is performed by the inverse of the foregoing procedure.
FIG. 11 is a view which illustrates another example of the structure of a conventional control rod driving apparatus. In this example, a connection pipe 30 is connected to a control rod 11 through a joining member 31. A drive piston 32 is disposed at the lower end of the connection pipe 30. The drive piston 32 constitutes a piston cylinder structure in association with a piston tube 33 and a cylinder tube 34. When hydraulic pressure is applied to an insertion port 35, drive water is allowed to pass through a passage designated by an arrow 36 so as to act on the lower surface of the drive piston 32. Thus, the drive piston 32 is pushed upwards. Therefore, the connection pipe 30 is moved upwards so that the control rod 11 is inserted into a reactor core. The position of the inserted control rod 11 is fixed because a control-rod position fixing finger 38 is received in a Groove 37 formed in the surface of the connection pipe 30. Therefore, the control rod 11 is fixed at step positions, the intervals of which correspond to the positions at which the grooves 37 are formed.
When the control rod 11 is withdrawn from the reactor core, hydraulic pressure is applied to a withdrawal port 39. Driving water is allowed to pass through a passage designated by an arrow 40 to pass through a hole 41 formed in the upper portion of the piston tube 33 and pass through a space between the piston tube 33 and the connection pipe 30 so as to act on the top surface of the drive piston 35. As a result, the drive piston is pushed downwards. On the other hand, a portion of the driving water is allowed to pass through a passage designated by an arrow 42 so as to act on the lower surface of the piston 43 and push the piston 43 upwards. Further, the control-rod position fixing finger 38 formed integrally with the piston 43 is moved upwards and widened by a guide member 44 so as to be separated from the groove.
If the control rod 11 is rapidly inserted into the reactor core during an emergency for the reactor, high-pressure water accumulated in an accumulator, not shown, is supplied to the insertion port 35 so as to rapidly push up the drive piston 32 and the connection pipe 30. Thus, the control rod 11 is inserted into the reactor core to cope with the emergency.
In a conventional BWR, either of the control-rod drive apparatuses respectively shown in FIGS. 10 and 11 is employed for all units constituting the reactor core without using the two types of the apparatus in a combined manner. The reason for this is that the combination necessitates the power source and the hydraulic pressure source as a drive source. Further, two types of control rod driving apparatus must be used because of differences in the control methods, thus causing the system's structure to be complicated. Therefore, an economical disadvantage arises.
A hydraulic pressure supply system in a conventional BWR arranged in a case where a hydraulic piston drive method as shown in FIG. 11 is employed will now be described.
FIG. 12 illustrates the schematic structure of a hydraulic pressure supply system in a conventional example. The piping structure is arranged in such a manner that a hydraulic-pressure supply portion 100 comprises a pump 101, a flow meter 102, a flow-rate adjustment valve 103, a pressure-adjustment valve 104 and a stabilizing circuit 105. The stabilizing circuit 105 comprises two systems of electromagnetic valves 106 and 107. One hydraulic-pressure supply portion 100 is provided for one atomic reactor plant. Pipes represented by pipes 109, 110, 111 and 112 are connected from the hydraulic-pressure supply portion 100 to a hydraulic-pressure control unit 108 which has pipes corresponding to those in the control rod driving apparatus 1. Water flows in the hydraulic-pressure supply portion 100 and in each pipe are designated by arrows.
The pipe 109 is a charging pipe for an accumulator 113 which acts when the control rod is inserted to cope with an emergency so that the accumulator 113 is charged with high-pressure water. The accumulator 113 includes a piston 114. The lower portion of the piston 114 is connected to a nitrogen container 116 through a pipe 115. High-pressure nitrogen gas is enclosed in the nitrogen container 116. Reference numerals 117 and 118 respectively represent a scram inlet valve and a scram outlet valve which are closed in a usual state so that the accumulator 113 is maintained at a high pressure state. In response to a control rod emergency insertion signal, the valves 117 and 118 are opened so that the high-pressure water in the accumulator 113 flows through an insertion pipe 119 connected to the lower surface of a drive piston of the control rod driving apparatus 1 so as to flow in the control rod driving apparatus 1. On the other hand, waste water discharged through the top surface of the drive piston flows to a withdrawing pipe 120 to flow through the scram outlet valve 118 so as to flow to a discharge container 121. As a result, a control rod is inserted into the reactor core to cope with the emergency.
The pipe 110 is a pipe for supplying water for driving the control rod when the output from a reactor is adjusted, the pipe 110 being connected to a direction-control circuit 126 composed of four electromagnetic valves 122, 123, 124 and 125 disposed in the hydraulic-pressure control unit 108. The direction-control circuit 126 acts to change over the hydraulic-pressure supply line in accordance with insertion/withdrawal of the control rod when a pair of two electromagnetic valves is opened. The driving water flows through the electromagnetic valve 122 and an insertion pipe 119 to be supplied to the lower surface of the drive piston of the control rod driving apparatus 1. On the other hand, discharged water from the top surface of the drive piston flows through the withdrawal pipe 120 and the electromagnetic valve 124 so as to be discharged from the hydraulic-pressure control unit 108 through a water-discharge pipe 112, the discharged water then being joined together the pipe 111. When the control rod is withdrawn, the electromagnetic valves 123 and 125 are opened. The driving water flows through the electromagnetic valve 125 and the withdrawing pipe 120 so as to be supplied to the top surface of the drive piston. On the other hand, discharged water from the lower surface of the drive piston flows through the insertion pipe 119 and the electromagnetic valve 123 and is discharged from the hydraulic-pressure control unit 108 through the water-discharge pipe 112, the discharged water being then joined together the pipe 111.
The pipe 111 is a pipe for water for cooling the control rod driving apparatus 1 so that cooling water, the pressure of which is adjusted, always flows through the insertion pipe 119 to flow in the control rod driving apparatus 1.
The electromagnetic valves 106 and 107 of the stabilizing circuit 105 are opened in a usual state so that water of a quantity required for the insertion of the control rod flows through the electromagnetic valve 106 and water of a quantity required for the withdrawal of the same flows through the electromagnetic valve 107. As a result, water flows in a cooling-water header 127 as a portion of cooling water. In the stabilizing circuit 105, the electromagnetic valve 106 is closed when the control rod is inserted in a usual state so that water of a quantity, which is the same as the quantity of water flowing through the electromagnetic valve 106, flows to the control rod driving apparatus 1. When the control rod is withdrawn, the electromagnetic valve 107 is closed so that water of a quantity, which is the same as the quantity of water flowing through the electromagnetic valve 107, flows to the control rod driving apparatus 1. As a result, the stabilizing circuit 105 stabilizes the pressure of drive water.
The conventional control rod driving apparatus of the structure described above is required to remove its electric motor and spool piece and decompose the spool piece by a predetermined number in one year in order to periodically change the shaft sealing packing. The operations for removing the electric motor and the spool piece are performed in a lower portion of the reactor pressure vessel, thus causing a possibility of radiation exposure for operators due to reactor water having a high radiation dose.
A great number of people and a large amount of manufacturing labor are required to complete a series of the operations, and therefore, periodical inspection period cannot be completed in a short time. What is worse, the shaft sealing packing undesirably enlarges the start torque due to coagulation between the shaft sealing packing and the rotational shaft if the ball spindle is left for a long time without being rotated. In this case, there is a possibility that operation of the electric motor cannot be performed smoothly. All electric motors must therefore be inspected at each periodical inspection of the reactor in order to inspect the operations of the electric motors. Also the foregoing fact inhibits the periodical inspection being completed in a short period of time.
It should be noted that the reactor core is designed in such a manner that power-adjustment units are previously determined, only their control rods are moved in the reactor core and the control rods for the residual units are completely removed from the core when the reactor is operated for the purpose of lightening the labor for operators and of reducing the fuel consumption cost.
It is preferable that the control rods for the power-adjustment units are of a type capable of fine motion in the core and of precisely controlling the power from the reactor. On the other hand, the control rods for the units except the power-adjustment units must have a function with which they can be rapidly inserted into the core to shutdown the reactor in an emergency without a necessity of having a capability of the precise movement.
In order to improve the operational facility and to reduce the cost of the fuel, it is advantageous for a reactor of a type having the foregoing core structure to dispose a driving apparatus suitable for the functions required for the control rods.
However, the same drive method is, at present, employed in the control rod driving apparatus for all units, and it can be said that the optimum arrangement has not yet been made available. For example, the conventional control rod driving apparatus shown in FIG. 10 is able to precisely adjust and move the control rod by controlling the rotational angle of the spindle thereof. Therefore, the apparatus has a structure suitable to serve as the control rod driving apparatus for the power-adjustment unit. On the other hand, the control rod driving apparatus shown in FIG. 11 basically employs the step drive. Therefore, it is not an optimum structure to meet a desire of precisely driving the power-adjustment units, but it is preferable that the same is used in units except the power-adjustment units. Hence, it is most preferred to use the two types of the control rod driving apparatuses in a combined manner in the core to meet the corresponding objects. However, the combined use requires using both the electric power source and the hydraulic pressure source because different drive methods are employed. Since also the control methods are different between these two types, two types of control apparatuses are required, thus resulting in the system structure becoming too complicated. What is worse, the cost reduction capability is inferior to the case where the single-method control rod driving apparatuses are used. Thus, the combined use has not been employed at present.